The toughness of steel is evaluated using mainly the Charpy test. In recent years, a crack tip opening displacement test (hereinafter referred to as a CTOD test) has often been used as a method for evaluating fracture resistance with high precision for thick steel plates for use in structures. In this test, initiation resistance to brittle fracture is measured by subjecting a test specimen having a fatigue precrack in a toughness evaluation portion to a low-temperature bending test and measuring the crack tip opening displacement (plastic strain) immediately before fracture.
Welding used for applying thick steel plates to structures is multipass welding. It is known that a multipass weld heat affected zone (hereinafter referred to as a multipass weld HAZ) includes a very low toughness zone (hereinafter referred to as ICCGHAZ: Inter Critically Coarse Grain Heat Affected Zone). The ICCGHAZ includes an island martensite (MA: Martensite-Austenite Constituent) microstructure in a coarse matrix microstructure, formed by reheating a zone having a coarse microstructure (CGHAZ: Coarse Grain Heat Affected Zone) in the vicinity of a weld line formed by a previous weld pass to a ferrite+austenite two-phase region in the weld pass of the next layer.
A steel plate is basically tested over the entire thickness in a joint CTOD test. Thus, in the joint CTOD test of a multipass weld HAZ, an evaluation zone into which a fatigue precrack is introduced includes an ICCGHAZ microstructure. The joint CTOD characteristics determined in the joint CTOD test are controlled by the toughness of the most brittle zone of the evaluated zone although the most brittle zone is very small. Thus, the joint CTOD characteristics of a multipass weld HAZ reflect not only CGHAZ microstructure toughness but also ICCGHAZ microstructure toughness. Thus, the improvement of the joint CTOD characteristics of a multipass weld HAZ requires the improvement of ICCGHAZ microstructure toughness.
Known techniques for improving heat affected zone (hereinafter also referred to as HAZ) toughness include suppression of austenite grain coarsening of CGHAZ using finely-dispersed TiN and the use of TiN ferrite transformation nuclei.
Other known techniques include suppression of austenite grain growth due to dispersion of REM oxysulfide formed by the addition of REM, suppression of austenite grain growth due to dispersion of Ca oxysulfide formed by the addition of Ca, and a technique using the ferrite nucleation ability of BN and oxide dispersion in combination.
For example, a technique for suppressing coarsening of an austenite microstructure in HAZ using REM and TiN particles is proposed in Patent Literatures 1 and 2. A technique for improving HAZ toughness using CaS and a technique for improving base material toughness by hot rolling are proposed in Patent Literature 3.
As a measure to prevent a decrease in ICCGHAZ toughness, a technique for increasing base material strength by decreasing the C and Si contents to suppress the formation of MA and by adding Cu is proposed (for example, Patent Literature 4). A technique for improving HAZ toughness by using BN as ferrite transformation nuclei in a high heat input heat affected zone to make a HAZ microstructure finer is proposed in Patent Literature 5.